一种新型实体壳单元及其板料冲压全工序仿真算法

发布时间：2018-02-14 20:33

本文关键词： 实体壳单元自适应网格加密板料冲压全工序仿真闭锁现象稳定化　出处：《华中科技大学》2016年博士论文　论文类型：学位论文

【摘要】：近年来,有限单元法已经成为一种重要的数值模拟手段,基于有限单元法的数值模拟软件已经在板料冲压领域得到了广泛应用。现有的商业有限元数值模拟软件主要采用薄膜单元和壳单元进行板料冲压过程模拟。但是,对于以汽车结构件为代表的厚板件,薄膜单元和壳单元并不能准确模拟板料变形。这是因为薄膜单元和壳单元都忽略了沿板料厚度方向的正应力,且无法体现板料的双面接触效果。为了弥补薄膜单元和壳单元存在的不足,本文提出了一种新的实体壳单元模型,建立了基于该单元的动力显式算法,并成功开发出了一套实体壳单元板料冲压全工序仿真算法。该算法既能有效反映成形过程中模具对板料的挤压效果,又能准确描述沿板料厚度方向的应力分布,显著改善了现有板料冲压有限元数值模拟软件在非平面应力成形问题中表现失真的问题,大大提高了板料冲压成形仿真和回弹预测的准确性。本文提出了一种无闭锁现象和沙漏模式的新型实体壳单元。通过引入1个EAS(Enhanced Assumed Strain,改善拟应变)内部变量,消除了单元的体积闭锁和厚向闭锁。采用ANS (Assumed Nature Strain,假设自然应变)四点插值方法,克服了单元的梯形闭锁和横向剪切闭锁。为了有效抑制面内单点积分引起的沙漏模式,提出了对应变转换矩阵和协变应变-位移矩阵Taylor展开,并对二者乘积进行投影和重构的方法,构建了不产生过刚现象的物理稳定化方法。采用若干经典线性算例验证了该实体壳单元的计算精度和收敛速度。建立了动力显式求解格式的实体壳单元成形仿真算法,解决了板料拉深、弯曲、翻边整形等大变形动态接触问题仿真中的收敛性难题。提出了内部变量的显式凝聚方法,有效减少了数据存储量。为了提高成形仿真的计算效率,提出了实体壳单元的自适应网格加密技术,并采用沙漏应力的继承保证了单元的稳定性。推导了服从Yld91和Y1d2004-18p屈服准则的弹塑性本构模型,为准确描述各向异性板材的变形行为提供了前提条件。基于建立的实体壳单元模型,开发了一套完整的板料冲压全工序仿真算法,并将该算法成功地应用于汽车铝合金结构件等厚板零件的成形、切边冲孔及回弹模拟。为了提高冲裁边界的尺寸精度,切边冲孔算法采用了网格加密的方法,并通过向前推进网格调整法和网格优化等措施,避免了畸形单元的产生。基于无模具的静力隐式回弹仿真算法,推导了隐式格式的单元稳定化方法,消除了刚度矩阵的秩缺陷,保证了回弹预测的准确性。多个算例表明,开发的实体壳单元板料冲压全工序仿真算法能获得较高的计算精度,基本达到了实用化水平。
[Abstract]:In recent years, the finite element method has become an important means of numerical simulation. The numerical simulation software based on finite element method has been widely used in the field of sheet metal stamping. The commercial finite element numerical simulation software mainly uses thin film element and shell element to simulate sheet metal stamping process. The film element and the shell element can not accurately simulate the deformation of the sheet metal for the thick plate represented by the automobile structure. This is because both the film element and the shell element ignore the normal stress along the direction of the thickness of the sheet metal. In order to make up for the shortage of thin film element and shell element, a new solid shell element model is proposed, and a dynamic explicit algorithm based on this element is established. A set of simulation algorithm for the whole process of solid shell unit sheet stamping is developed, which can not only effectively reflect the extrusion effect of the die on the sheet metal in the forming process, but also accurately describe the stress distribution along the direction of the thickness of the sheet metal. The problem that the finite element numerical simulation software of sheet metal stamping in the non-plane stress forming problem is obviously improved is improved. The accuracy of sheet metal stamping simulation and springback prediction is greatly improved. In this paper, a new type of solid shell element without locking phenomenon and hourglass mode is proposed. By introducing a EAS(Enhanced Assumed training, the internal variable is improved. The volume and thick latchup of the element are eliminated. The trapezoidal locking and transverse shear latchup are overcome by using ANS sumed Nature training (assuming natural strain) four-point interpolation method. In order to effectively suppress the hourglass mode caused by in-plane single point integration, the trapezoidal latchup and transverse shear latchup of the element are overcome. The Taylor expansion of strain transformation matrix and covariant strain-displacement matrix is proposed, and the method of projecting and reconstructing the product of the two matrices is presented. In this paper, a physical stabilization method without overstiffness is constructed, and some classical linear examples are used to verify the accuracy and convergence speed of the solid shell element. A dynamic explicit solution scheme is established to simulate the forming of the solid shell element. The convergence problem in the simulation of large deformation dynamic contact problems such as drawing, bending and flanging of sheet metal is solved. The explicit condensation method of internal variables is proposed, which effectively reduces the data storage. In order to improve the computational efficiency of forming simulation, An adaptive mesh encryption technique for solid shell elements is proposed, and the stability of the element is guaranteed by inheriting the hourglass stress. An elastoplastic constitutive model is derived, which is based on the yield criteria of Yld91 and Y1d2004-18p. In order to accurately describe the deformation behavior of anisotropic sheet metal, a complete simulation algorithm for sheet metal stamping process is developed based on the established solid shell element model. The algorithm is successfully applied to the forming, edge-punching and springback simulation of automotive aluminum alloy parts with equal thickness plate. In order to improve the dimension accuracy of blanking boundary, the edge-cutting punching algorithm adopts the method of mesh encryption. By advancing forward mesh adjustment method and mesh optimization, the generation of deformable element is avoided. Based on the static implicit springback simulation algorithm without mould, the element stabilization method of implicit scheme is derived, which eliminates the rank defect of stiffness matrix. The accuracy of springback prediction is ensured. Several examples show that the developed simulation algorithm for the whole stamping process of solid shell plate can obtain high calculation accuracy and reach the practical level.
【学位授予单位】：华中科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG386;TP391.9

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